Slewing bucket-wheel reclaimers are the most common type of reclaimers used in the iron ore and coal industries. Another common type of reclaimer is the bridge reclaimer.
Bucket-wheel reclaimers are a high cost mining asset. Individual machine cost may exceed $30 M, with supporting stockyard infrastructure adding significant cost. Relatively small improvement in reclaimer productivity will provide a significant economic benefit to the business. As an example of the economic benefit that can be achieved is given below:                Ship load time is 20 hours for 200 kt at 10,000 tph.        A 2.5% reclaim rate improvement (10,000 tph to 10,250 tph) reduces ship load time by approximately 30 minutes.        Based on 300 machine production days per annum this equates to a 150 hour reduction in machine operating time.        Sustained rate improvement would provide >5,000 t per day increased machine production.        Based on 300 days machine utilisation per annum, this equates to a production opportunity of more than 1.5 Mt per annum.        
Slewing bucket-wheel reclaimers operate in the following manner. The stockpile is reclaimed in a series of ‘Benches’ where each bench defines a layer of the stockpile, as illustrated in FIG. 2. The height of each layer depends on the bucket-wheel size, with a typical bench height being equal to the bucket-wheel radius (5.0 meters) and a maximum bench height being 0.65 of the diameter (6.5 meters). The reclaimer starts at the top bench of a previously stacked stockpile and reclaims the bench in a series of radial cuts by slewing (swinging) the bucket-wheel across the face of the stockpile, as shown in FIG. 2.
At the end of each face cut, the reclaimer travels forward (Step Advance) a short distance (typically 1.0 m for a 5.0 m bucket-wheel), and then begins the next cut. The rate of reclaiming is controlled during the face cut by adjusting the speed of the slew motion. The general formula for reclaim rate when digging a full height bench face, in cubic meters per second, at any point along the face cut is:Face Height (meters)×Face Cut Depth (meters)×Radial Slew Speed (meters per second).
Where: Face Cut Depth=Cosine (Slew Angle)×Step Advance Distance
The actual rate will depend on the shape of the stockpile at the bucket-wheel face.
The overwhelming majority of bucket-wheel reclaimers are fitted with power-based reclaim rate controllers. Power-based reclaim rate controllers derive an implied reclaim rate based on the digging power of the bucket-wheel.
Reclaiming is undertaken in order to move product from a stockpile to a destination, be it a train, ship or another stockpile via a transport system.
In general terms, minimum cost to move the product is achieved by transporting the product at the maximum rate supported by the transport equipment. The maximum rate supported by transport equipment is determined by the maximum volume rate. For example:                1. The maximum transport rate for a belt conveyor is usually limited by the volume that can be handled without spillage over the edges of the belt.        2. The maximum transport rate for a transfer chute is limited by the volume that can pass through the chute without blockage.        
Although volume is normally the limiting factor, current reclaim rate controllers use an implied reclaim weight rate controller (controls in tonnes per hour). One of the disadvantages of prior art reclaim rate controllers is the inability to control the reclaim rate in terms of volume. This results from the inability to measure the volume rate at the bucket-wheel. Inability to control the volume rate means that they cannot achieve the maximum transport volume rate.
Whilst volume is generally the limiting factor for transport equipment, there are cases where weight is also a limiting factor. For example, a conveyor trestle may have a weight limitation that overrides the volume limitation of the belt conveyor itself. In these cases, maximum transport efficiency is achieved by maintaining a consistent transport rate. Current reclaim rate controllers have poor performance in terms of rate fluctuation. This is due to their inability to accurately measure the reclaim rate based on implied measurement techniques. This is further explained in the following section.
In the case where there is a requirement to reclaim at a low rate, the inaccurate rate measurement of existing rate controllers results in incorrect rate and high rate fluctuations. Power based rate controllers are unable to determine the stockpile edges at low reclaim rates and often require operator intervention to set fixed reclaim slew range limits.
Due to the low cut depth at the outer slew region of a stockpile face cut, it is advantageous to finish the cut early for several cuts before cleaning up the ridge with a single longer cut. This practice is known a ‘Waltz Step’ of ‘Clean-Up Pass’. However ‘Waltz Step’ reclaiming is rarely used with power based rate controllers, due primarily to their inability to adequately control the rate during the step changes in cut depth between the current face and the outer ridge.
Current reclaim rate control systems use implied methods to measure the reclaim rate, including digging energy (bucket-wheel current) or digging force (bucket-wheel torque). The achieved reclaim rate depends on the bucket-wheel digging efficiency (cubic meters per unit of energy/force) which is affected by a range of parameters including:                Product Type (particularly the granule size)        Product Mineral Composition (mine and section of ore body)        Product Density (variation of source product)        Moisture Content (from rain or dust suppression sprays)        Secondary Processing (combinations of crushing, screening and blending)        Bucket-wheel Cutting Efficiency for Different Products        Bucket-wheel Cutting Efficiency for Clockwise vs. Counter Clockwise        Bucket-wheel Cutting Efficiency due to wear        Product Compaction (time since stacked)        Stacking Pattern        No Load Current/Torque Drift        Non Linear Load to Rate Relationship        
As the state of the stockpile is unknown, it is not possible to provide compensation for these factors. This results in less than optimal reclaim rates. Efforts to improve reclaimer productivity are limited by the reclaim rate measurement error.
Various systems attempt to improve the accuracy of the implied reclaim rate by use of single point or 2D radar sensors. These systems may collectively be referred to as ‘predictive rate controllers’. Predictive rate controllers use 2D radar scanners to predict the approximate volume that will be reclaimed by the bucket-wheel. Predictive volume based systems perform a vertically orientated 2D scan of the stockpile face, with the third dimension being provided by the slew motion. The 2D scanner is located at a position ahead of the bucket-wheel.
An example of a prior art predictive rate controller utilising a 2D radar scanner is the system marketed by Indurad (Germany) as a ‘Bucket-wheel Excavator Predictive Cutting Control’. The control is described to provide customer benefits of ‘Predictive volume flow information and operator assistance’.
The radar scanners used in existing predictive systems are based on 77 GHz vehicle collision avoidance radar units. The combination of field of view (FOV) angle resolution (typically 4 degrees) and target distance accuracy (typically +−150 mm) results in inability to measure the stockpile face volume, particularly when the bucket-wheel cut depth is less than one meters (1.0 m).
During reclaiming operations, the stockpile area around the bucket-wheel will collapse and flow as product is removed. Accurate measurement of the reclaim rate requires that the volume in the area abutting the bucket-wheel be continuously measured. The 2D nature of the predictive volume scanning system means that the actual volume being reclaimed by the bucket-wheel cannot be measured. Instead, the reclaim volume is predicted. Collapsing and dynamic movement of the stockpile due to flow of product is not measured.
Predictive volume systems are typically used for operator assistance on manually operated reclaimers or as the theoretical (feed forward) speed of an implied (current/torque) reclaim rate controller. Whilst predictive volume systems improve the performance of an implied rate controller, the control performance is still affected by the same factors as the standard implied rate controller.
Prior art use of 3D laser scanning for stackers and reclaimers is described in European patent EP1278918, also published as US 2005/0246133. This prior art document is referred to hereinafter as P2.
The system described in P2 scans the stockpile to determine the stockpile shape for the purpose of controlling the movement of the reclaimer to the facing up position and to determine the slewing range of the bucket-wheel during reclaiming.
One of the problems that P2 seeks to overcome is the inaccuracies in the stockpile model that occur when using a 2D scanner where the stockpile shape is initially determined by way of a measurement pass of the bucket-wheel device and the 2D scanner, and then after the removal or stacking process is initiated the controller calculates a provisional stockpile model.
However this 2D system cannot detect changes of the stockpile shape which occur during the operation of the bucket-wheel device, for example, due to rainfall and the natural downslide processes or the like, as well as slides or downslides triggered by the removal process itself. P2 overcomes these problems by scanning the stockpile using a 3D laser scanner to determine the actual stockpile shape independently of the operation of the bucket-wheel device. The system described in P2 includes GPS receivers to provide accurate position information for the bucket-wheel reclaimer and/or the bucket-wheel itself. A claimed benefit of the system described in P2 is that the stockpile shape may be captured without carrying out a measurement pass and that bumping into the stockpile is avoided.
The system described in P2 is not able to measure the reclaimed volume at the bucket-wheel as the area abutting the bucket-wheel is not scanned. Furthermore there is no disclosure or suggestion in P2 of calculating a reclaim volume of material that will be cut from the stockpile face, based on the shape of the excavation tool and the 3D stockpile shape, to determine a cut reclaim volume rate. In fact there is no reference whatsoever in P2 to either volume measurement or reclaim rate control. The described control function is to position the bucket-wheel device in dependence on the measured stockpile shape, in order to optimise initial face-up positioning of the bucket-wheel and to control the bucket-wheel swing range based on the shape of the stockpile.
A commercial implementation of P2 was developed by iSAM AG (Germany) and is marketed by FL Smidt as the ‘iSAM Automation System for Stacker Reclaimers.’ The referred commercial implementation of P2 uses bucket-wheel power based implied reclaim rate control.
The present invention was developed with a view to providing a 3D volume rate controller method and apparatus which is less susceptible to the above-noted problems and disadvantages of the prior art implied reclaim rate controllers and predictive rate controllers.
References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.